The present invention relates to an optical fiber and an optical amplifier; and, more particularly, to a holmium (Ho3+)-doped optical fiber and an optical amplifier operable in a wavelength ranging from 1,340 nm to 1,410 nm by using the Ho3+-doped optical fiber.
The wavelength range of a low optical loss is approximately 1,200xcx9c1,700 nm in a conventional transmission silica optical fiber, whereby the broad wavelength range, i.e., approximately 500 nm range, is capable of being used in an optical transmission. Thus, it is most important thing to make an optical amplifier operable in this broad wavelength range. Up to now, there have been developed ion-doped optical amplifiers of which the wavelengths ranges are 1,300 nm, 1,450 nm, 1,530 nmxcx9c1,610 nm and 1,650 nmxcx9c1,680 nm, respectively. Though a rare earth ion-doped optical amplifier is capable of amplifying an optical signal having a specified wavelength band owing to characteristics of the rare earth ion and a host material, it is still demanded to develop the broad band optical amplifier in order to make use of the broad wavelength range of the low optical loss.
There has not been introduced a rare earth-doped optical amplifier representing a gain property in the wavelength ranging from 1,340 nm to 1,410 nm. However, there was announced an optical laser oscillated at 1,380 nm wavelength by using a holmium ion (Ho3+)-doped fluoride optical fiber, as described in M. C. Brierly, P. W. France, C. A. Miller, xe2x80x9cLasing at 2.08 xcexcm and 1.38 xcexcm in a Holmium-Doped Fluoro-Zirconate Fiber Laserxe2x80x9d, Electronics Letters, Vol.204, No.9, pp. 539xcx9c540 (1988). In this case, a pumping wavelength was shorter than 530 nm.
Furthermore, there has been no research result that an upconversion pumping is possible at the wavelength ranging from 900 nm to 1,000 nm when holmium ions are doped into an amorphous optical fiber material. The upconversion can be observed at 650 nm wavelength in an experiment using the holmium-doped sample, as described in D. S. Funk, S. B. Stevens and J. C. Eden, xe2x80x9cExcitation Spectra of the Green Ho:Fluorzirconate Glass Fiber Laserxe2x80x9d, IEEE Photonics Technology Letters, Vol.5, No.2 (1993).
There was a research in which it is effective to dope ytterbium ions (Yb3+) in order to enhance a pumping efficiency through the upconversion pumping at the wavelength range of 900 nm to 1,000 nm. However, as a concentration of the Yb3+ increases, an energy transfer from the Ho3+ to the Yb3+ is generated so that a fluorescence life time of a 1,370 nm fluorescence emission level is shortened, as described in X. X. Zhang, P. Hong, M. Base and B. H. T. Chai, xe2x80x9cHo3+ to Yb3+ Back Transfer and Thermal Quenching of Upconversion Green Emission in Fluoride Crystalsxe2x80x9d, Applied Physics Letters, Vol.63, No.19, pp. 2,606xcx9c2,608 (1993). Thus, it is necessary to determine an optimized doping concentration of the Yb3+ for preventing the energy transfer from the Ho3+ to the Yb3+. In case of applying the 1,370 nm band of the holmium to the optical amplifier, any optical waveguide scheme has not been suggested yet to reduce amplified spontaneous emission (ASE) of 540 nm and 750 nm fluorescence, which are emitted simultaneously with 1,370 nm fluorescence.
In case of fabricating the optical amplifier operable in an aimed wavelength band by using (5S2, 5F4)xe2x86x925I5 transition of the Ho3+, there are several problems as followings.
First, the (5S2, 5F4) level of the Ho3+ is positioned at approximately 535 nm wavelength. Thus, if a pumping source is shorter than 540 nm wavelength, it is difficult to secure the suitable pumping source in practice. Furthermore, a splicing loss to the transmission silica optical fiber also increases in case of shortening a cutoff wavelength of the optical waveguide.
Second, since a branching ratio of the 540 nm fluorescence and the 750 nm fluorescence which are emitted from the (5S2, 5F4) level is higher than that of the 1,370 nm fluorescence, the gain of the 1,370 nm fluorescence is saturated at a low gain value because of the ASE.
Third, when an absorption band of the host material at short-wavelength side is overlapped with the (5S2, 5F4) level, the energy of the (5S2, 5F4) level is absorbed due to a band gap transition of the host material, thereby decreasing the fluorescence lifetime and the fluorescence intensity remarkably.
It is necessary to use a low phonon energy host to minimize the decrease of the fluorescence lifetime, because an energy gap between the (5S2, 5F4) level and next-lower-lying level, i.e., 5F5, is approximately 2,800 cmxe2x88x921 so that a non-radiative relaxation may occur owing to a multiphonon relaxation mechanism. In addition, in order to enhance gain property at 1,370 nm band, the fluorescence lifetime of the (5S2, 5F4) level should be prolonged and a refractive index should be small because the splicing loss induced from the difference of the refractive indices should be minimized in case of being contact with the transmission silica optical fiber. Since the fluorescence lifetime of the (5S2, 5F4) level is shortened by the absorption due to a vibration of hydroxyl radical (OHxe2x88x92), it should be further satisfied to remove the hydroxyl radicals from the optical waveguide with ease.
It is, therefore, an object of the present invention to provide a holmium (Ho3+) doped optical fiber for use in an optical amplifier.
It is another object of the present invention to provide an optical amplifier operable in 1,340 nm to 1,410 nm incorporating therein the Ho3+-doped optical fiber.
In accordance with one aspect of the present invention, there is provided an optical fiber composition, wherein a representative phonon energy of a host material is less than 600 cmxe2x88x921 and an absorption band at short-wavelength side is less than 530 nm, the optical fiber composition comprising: a core including holmium ions less than 0.5 mole %; and a clad including ions selected from the group consisting of transition metal ions, rare earth ions and a combination thereof.
In accordance with another aspect of the present invention, there is provided an optical fiber, wherein a representative phonon energy of a host material is less than 600 cmxe2x88x921 and an absorption band at short-wavelength side is less than 530 nm, the optical fiber comprising: a core including holmium ions less than 0.5 mole %; and a clad including ions selected from the group consisting of transition metal ions, rare earth ions and a combination thereof.
In accordance with further another aspect of the present invention, there is provided an optical amplifier for amplifying an optical signal, comprising: a pump source for generating a pumping light; a wavelength division multiplexing (WDM) coupler for multiplexing an input optical signal and the pumping light; and an optical fiber including a core and a clad, wherein the core has holmium ions less than 0.5 mole % and the clad has ions selected from the group consisting of transition metal ions, rare earth ions and a combination thereof.